Method of making self-baking continuous electrodes



May 24, 1960 H. SCHMITT ET AL 2,937,980

' METHOD OF MAKING SELF-BAKING con'rmuous ELECTRODES Filed Jan. 23,195"! 2 Sheets-Sheet 1 INVENTORS ,4 SJCHM/TT' mer TOMA ATTORNEYg May 24,1950 ,'scH ET AL METHOD OF MAKING SELF-BAKING CONTINUOUS ELECTRODESFiled Jan. 23, 1957 2 Sheets-Sheet 2 INVENTOR5 H4: J'cHM/rr ///.or TOM/9BY r" E a y! I W W ATTORNEY United States Patent METHOD OF MAKINGSELF-BAKING CONTINUOUS ELECTRODES Hans Schmitt and Kurt Toma,Rheinfelden, Baden, Germany, assignors, by mesne assignments, toElektrokemisk A/S, Oslo, Norway, a corporation of Norway Filed Jan. 23,1957, Ser. No. 635,874

Claims priority, application Switzerland Jan. 24, 1956 Claims. (Cl.204-67) In electrolytic cells for the production of aluminum in a fusedsalt bath two types of anodes are used: The selfbaking anodes of theso-called Soderberg type which are produced continuously according totheir consumption in the cell or the so-called pre-baked anodes, madefrom pressed carbonaceous material and baked in a separate furnace,which have to be replaced after consumption by new pre-baked anodes, sothat the anodes are handled discontinuously.

The cells provided with pre-baked anodes have the advantage of a loweranodic voltage drop compared with the cells with self-baking anodes. Thedifference is mainly caused by the lower specific resistance of thepressed and pre-baked anodes. The specific resistance of pre-bakedanodes has a value of 50 to 70.10" ohm.cm. .cm." at a temperature of 20C. whereas the baked S'oderberg paste has a specific resistance of 80 to110.10- ohm.cm. .cm.- Furthermore. the voltage drop between the contactstuds and the carbon anode is generally lower in the pro-baked anodesthan in the Sbderberg anodes, as the contact studs are fixed into thepre-baked anodes outside the cell, where this operation can be donecarefully, whereas with Stiderberg anodes the contact studs must bedriven into the carbonaceous mass on the pot during the operation insuch a way that they may be loosened and pulled out when they havereached their deepest position. Moreover the smallest distance betweenthe lower end of the contact stud and the lower face of the pre-bakedanode just before replacing the latter is generally smaller than thecorresponding distance between the contact stud and the lower face ofthe Soderberg anode before pulling out the stud. That means that theaverage path of current from the stud to the lower face is generallyshorter in pre-baked anodes than in Stiderberg anodes.

All in all it is possible to operate cells with pre-baked anodes with ananodic voltage drop which is by 0.15 to 0.3 volt lower than in cellswith self-baking anodes. This voltage gain enables to make savings of0.5 to l kwh. per kilogram aluminum.

But nevertheless the Soderberg anodes are generally preferred especiallyin electrolytic cells of high current intensity as in this case it isnot necessary to prepare a plurality of anodes outside the cell, thedimensions of which anodes are limited through the pressing and bak ingprocess, and as it is possible to equip the cell with one large orat-most two anodes. It is known that the use of Soderberg anodespresents great advantages as to the construction of the anodic part ofthe cell and the operation of the same. A further advantage of the useof only one or two large anodes in each cell consists in the fact thatthe current is distributed more uniformly over the whole cross-sectionof the anode and that therefore the distance of the anode from thecathode layer is rather the same over the whole cross-section of theanode because of a more uniform consumption of the same. With aplurality of pre-baked anodes in one cell it is practically impossibleto distribute the current uni- Patented May 24, 1960 ice formly and tokeep all anodes at the same distance from the cathode layer.

This advantage of the Stiderberg anodes compared with the pre-bakedanodes is especially noticeable in cells which are operated with acurrent intensity of more than 60,000 amperes. It is known that in cellsof such a high current intensity electromagnetic forces are generatedwhich may cause a vaulting of the molten metal leading to troubles inthe normal operation. In a cell with a plurality of anodes the surfaceof the molten metal fluctuates more and in a greater extent than in acell with one or two large anodes, probably because of the differentdistance of the electrodes from the cathode layer and of the differentcurrent charging rates of the anodes. Also during the replacement ofconsumed anodes always troubles arise in the distribution of thecurrent; these troubles favour the vaulting of the moten metal. Also incells with Soderberg anodes the electromagnetic forces cause a vaultingof the molten metal but the vaulting is not as great as in the cellswith pre-baked anodes as the current is more uniformly distributed anthe operation continuous.

That is why in cells with pre-baked anodes the average distance of theanodes from the cathode must be greater than in Soderberg cells of thesame current intensity. This greater distance results in a higher bathvoltage without increase of current yield, as the increased vaulting ofthe molten metal causes a re-oxidation of the metal deposited on thecathode and therefore a decrease of the current yield.

With respect to the vaulting of the molten metal, cells of a currentintensity of 60,000 amperes and more with pre-baked anodes musttherefore be operated with a higher bath voltage than Stiderberg cellswith one anode.

The advantage of the lower anodic voltage drop in the cells with aplurality of pre-baked anodes is thereby practically nullified and whenoperating with very high current intensities of for example 80,000 or90,000 amperes the advantages of the Stiderberg cell predominate moreand more. Cells with pre-baked anodes and of such a high currentintensity must be operated generally with a the lower blocks by means ofa carbonaceous paste.-

After consumption of the lower blocks the contact studs must bedisplaced from the lower to the upper blocks. This may only be done whenthe joining carbonaceous paste between the blocks has been baked and hasthereby reached such a high conductivity that no inadmissible voltagedrop in the joint arises. It is the disadvantage of these continuousanodes from pre-baked blocks that no adhesive has yet been foundallowing an infallible joining of the pre-baked blocks and that there isalways the danger of an unsufiicient joining of the blocks or of anunsuflicient baking or coking of the adhesive which consists generallyof coke or carbon powder and pitch. This may lead to remainders (scraps)of the lower block falling down into the bath or to a high voltage dropin the joint.

The present invention relates to a method of making a continuous anodeto be used in electrolytic cells for the production of aluminum andavoiding the mentioned disadvantages of the known pre-baked orself-baking continuous anodes. According to this method a. continuousanode is made by joining and sticking together unbaked pressed, rammedor extruded blocks of carbonaceous material. The anode is made acontinuous one by setting up on the anode in the cell new unbaked blocksaccording to the consumption of the lower blocks and joining the unbakedblocks to the lower blocks by means of a carbonaceous adhesive, forexample Siiderberg paste. An adhesive having a lower viscosity than thecarbonaceous material of the blocks, so that it begins to flow at alower temperature, is preferred. Such a material can be better broughtinto the joints between the blocks as a material of a higher viscosityand fills also more thoroughly the holes on the surface of the blocks.The adhesive may be applied in liquid or sappy state by pouring into thejoints or by spreading over the surface. A carbonaceous material of ahigher viscosity than Soderberg paste may also be used as adhesive, forexample a material of the same composition as the material of the blocksor a similar material but with a lower pitch content; but such anadhesive must be applied by stamping instead of pouring.

The composition of the carbonaceous material (consisting for instance ofpulverized coke and coal-tar pitch) of the blocks which have to be usedfor making the anodes according to the invention corresponds to thecomposition of the normal and usual anodes before baking, that is to saythat the content of pitch as a binder amounts to about 16 to 20 percent.

Examples for the composition of the anodes (1) An anode materialconsisting of about 70 percent pitch coke and 30 percent purest coalcoke with an addi- .tion of pitch as a binder.

(a) Composition of the mixture:

Grain size ac- Content in Coke type cording to the percent of Tylersieve, the whole mm. mixture Pitch coke 1. 68 to 3. 36 8 Do 0.21 to 1.68 30 Do..- to 0.21 19 Forest coal coke 1. 68 to 3. 36 3 D0 0.21 to 1.(:8 8 D0 0 to O. 21 14 Hard pitctn. 18

(b) Specification of the hard pitch:

Softening point according to Kr'aimer-Sarnow C 86 Coking residue percent60 Insoluble in anthracene oil do 14 Insoluble in benzene do.. 44

(2) An anode material consisting of 55 percent pitch coke, 30 percentcoal coke obtained at high temperature and 15 percent pulverized anodescrap with an addition of solid pitch as a binder.

(a) Composition of the mixture:

I (b) Specification of the hard pitch as in Example 1.

, joining of the blocks.

4? Examples for the composition of the carbonaceous adhesive (So'derbergpaste) (1) A Sbderberg paste consisting of 70 percent pitch coke, 30percent purest coal coke and an addition of middle hard pitch as abinder.

(a) Composition of the mixture:

Content in percent of the whole mixture Grain size according to theTyler sieve, mm.

Coke type Pitch coke Do Purest coal coke...

Do Middle hard pitch (b) Specification of the middle hard pitch:

Softening point according to Kr'aimer-Sarnow ..C 76 Coking residue"percent.- 60 Insoluble in anthracene oil do 4 Insoluble in benzene do30 (2) A Soderberg paste consisting of pitch coke and hard pitch as abinder.

(a) Composition of the mixture:

Grain size according to the Tyler sieve, mm.

Content in percent of the whole mixture Coke type (b) Specification ofthe hard pitch:

The accompanying drawing shows as an example an electrolytic cell forthe production of aluminum provided with an anode according to theinvention. Fig. 1 shows a side view of the cell partly in section. Fig.2 shows the cell in a top view, the upper parts of the cell with thecurrent supply being not shown.

The numeral 1 designates the pot of the cell provided with a carbonlining. This pot is of known type. The anode made according to theinvention is composed of the blocks 2 of carbonaceous material. Thejoints 3 are filled with Siiderberg paste. The whole packet ofcarbonaceous blocks is surrounded and held by the iron frame 4, thesuspension of which being for the sake of simplicity not shown. Theanode slides downwards within this frame 4 according to its consumption.Before setting on new blocks the upper surface of the lower blocks iscovered with the StSderberg paste as adhesive for the The horizontaldistance between the blocks put side by side is preferably 5 to 20 mm.;the joints are also filled with Siiderberg paste.

in the example the blocks 2 forming the anode are staggered in thehorizontal plane. But it is also possible to arrange them verticallystaggered.

In the lower part of the anode the joined blocks are baked by the effectof the heat generated in the cell and the, Siiderberg paste is coked. Inthe upper part of the anode the blocks are still unbaked and theStiderberg paste not yet coked.

The current supply to the anode is ensured by means of contact studswhich are preferably of round section, made of steel and arrangedvertically; one of them is shown in Fig. 1, designated with the numeral5. Half cylindrical grooves on abutting faces of two blocks arrangedside by side form the hole for driving in the contact stud; this holeextends through all blocks set upon each other. The diameter of thishole is preferably 20 to 50 mm. larger than the diameter of the contactstud. The grooves are formed during pressing or ramming the blocks. Theholes may also be arranged in the interior of the blocks and formed forexample by means of a mandrel during the extrusion of the blocks. Afterdriving in the contact stud, the hole is filled up with Stiderbergpaste.

The contact studs are pulled upwards from time to time according to theconsumption of the anode, their lower extremity remaining always in thebaked part of the anode.

One may also use as contact studs slit tubes or tubes composed of twotube-halves, these tubes being provided with a metal insert spreadingthe two parts of the tube against the hole wall. Before pulling thetube-studs upwards the inserts are drawn out, so that the tube-studs mayeasily be loosened.

Of course the contact studs may also have another cross-section than acircular one, for example a square one.

A cell with an anode according to the invention may be put in operationin the same way as a cell with a Sliderberg anode:

Conducting elements are inserted between the bottom of the cell formingthe cathode and the lower extremity of the contact studs on the unbakedanode which rests on the said bottom. These elements connectelectrically the contact studs with the cathode, as the unbakedcarbonaceous material is non-conducting. This material is baked throughthe current heat generated in the conducting elements, so starting thecoking process of the anode. The conducting elements may be made ofiron, graphite or a baked carbonaceous material.

It is also possible to start with an anode the lower blocks of whichhave been pre-baked in their lower part outside the cell.

Tests with a cell equipped with an anode according to the invention haveshown that pressed blocks from unbaked carbonaceous material withSfiderberg paste as a sticking agent are thoroughly joined togetherduring the progressive coking, so that a suflicient mechanical andelectrical joint is ensured. The voltage drop in the joint is hardlyhigher than in the baked mass of the block and there are no remaindersand fragments falling down into the bath as it happens often in cellswith the known continuous pre-baked anodes.

In anodes according to the invention the blocks do not run during thecoking process under the influence of the heat in the cell as inStSderberg anodes. Because of their lower pitch content, the blockssoften without losing practically their shape.

Cells provided with anodes according to the invention have the advantageof a lower anodic voltage drop compared with the Soderberg cells, asthey allow to utilize the higher conductivity of pressed blocks, andlike cells with pre-baked anodes they have the advantage of working witha lower bath voltage. It is therefore possible to operate cells providedwith anodes according to the invention with an overall voltage which isby 0.15-0.3 volt lower than in Stiderberg cells, so that the consumptionof energy is decreased by about 0.5-1 kwh. per kg. of aluminum.

What we claim is:

1. The method of operating cells for the electrolytic production ofaluminum in fused fluorides containing bath using self-baking continuousconsumable anodes from carbonaceous material with metallic studs for thesupply of current, which method comprises repeatedly covering the top ofthe carbonaceous self-baking anode with a layer of carbonaceous paste ofsubstantially the same composition as the carbonaceous material of theanode and laying unbaked preformed blocks of carbonaceous material andof predetermined size and shape thereon without interrupting theoperation of the cell, the bottom of the anode being already baked andhaving substantially the electrolysis temperature and the top of theanode being unbaked, and bonding the unbaked preformed blocks side byside by means of carbonaceous paste of substantially the samecomposition as the carbonaceous material of the anode, the unbakedpreformed blocks being added to the top of the anode at a rate tocompensate for the progressive consumption of the bottom of the anode.

2. The method according to claim 1, wherein the carbonaceous paste usedfor repeatedly covering the top of the carbonaceous self-baking anodehas a lower viscosity than the carbonaceous material of the unbakedpreformed blocks being added to the top of the anode.

3. The method according to claim 1, wherein the unbaked blocks are madefrom a carbonaceous material consisting essentially of pulverized cokeand pitch.

4. The method according to claim 1, wherein the carbonaceous adhesiveconsists essentially of pulverized coke and pitch.

5. The method according to claim 1, wherein the assembled blocks aresurrounded by a metal frame and slide downward as a-unit along saidframe as the anode is consumed.

References Cited in the file of this patent UNITED STATES PATENTS2,728,109 Bonnet Dec. 27, 1955 2,739,113 Horsfield et al. Mar. 20, 19562,758,964 Liles Aug. 14, 1956 FOREIGN PATENTS 328,178 Italy July 31,1935 363,551 Italy Oct. 7, 1938 1,080,982 France Dec. 15, 1954 913,805Germany June 21, 1954

1. THE METHOD OF OPERATING CELLS FOR THE ELECTROLYTIC PRODUCTION OFALUMINUM IN FUSED CHLORIDES CONTAINING BATH USING SELF-BAKING CONTINUOUSCONSUMABLE ANODES FROM CARBONACEOUS MATERIAL WITH METALLIC STUDS FOR THESUPPLY OF CURRENT, WHICH METHOD COMPRISES REPEATEDLY COVERING THE TOP OFTHE CARBONACEOUS SELF-BAKING ANODE WITH A LAYER OF CARBONACEOUS PASTE OFSUBSTANTIALLY THE SAME COMPOSITION AS THE CARBONACEOUS MATERIAL OF THEANODE AND LAYING UNBAKED PREFORMED BLOCKS OF CARBONACEOUS MATERIAL ANDOF PREDETERMINED SIZE AND SHAPE THEREON WITHOUT INTERRUPTING THEOPERATION OF THE CELL, THE BOTTOM OF THE ANODE BEING ALREADY BAKED ANDHAVING SUBSTANTIALLY THE ELECTROLYSIS TEMPERATURE AND THE TOP OF THEANODE BEING UNBAKED, AND BONDING THE UNBAKED PREFORMED BLOCKS SIDE BYSIDE BY MEANS OF CARBONACEOUS PASTE OF SUBSTANTIALLY THE SAMECOMPOSITION AS THE CARBONACEOUS MATERIAL OF THE ANODE, THE UNBAKEDPREFORMED BLOCKS BEING ADDED TO THE TOP OF THE ANODE AT A RATE TOCOMPENSATE FOR THE PROGRESSIVE CONSUMPTION OF THE BOTTOM OF THE ANODE.